As a technology for producing glass sheets, the downdraw method has been publicly known, in which glass sheets are produced by forming a glass ribbon by causing molten glass to flow downward from a forming body and cutting the glass ribbon into predetermined dimensions. Typical examples of the downdraw method include an overflow downdraw method, a redraw method, and a slot downdraw method.
Forming methods using the downdraw methods of this type include, as basic steps of producing glass sheets, a forming step of forming a glass ribbon into a sheet-like shape (belt-like shape) having predetermined dimensions, an annealing step of annealing the glass ribbon so that internal distortion is removed; a cooling step of cooling the glass ribbon to about room temperature; and a cutting step of cutting the glass ribbon into predetermined dimensions.
Of those steps, at least the forming step and the annealing step are performed while the glass ribbon is moved downward.
In particular, of those steps, the annealing step is a step of removing internal distortion of the glass ribbon, and hence is an important step to determine product quality (performance) of the glass sheets. Specifically, when the internal distortion remains in the glass ribbon even after the annealing step, the internal distortion remains as it is also in the glass sheets produced by cutting the glass ribbon. As a result, a mechanical strength of the glass sheets may be significantly reduced, or disturbance of various characteristics, such as unintended birefringence, may occur. In particular, as for glass sheets for flat panel displays such as a liquid crystal display, the residual internal distortion causes the birefringence and the like. As a result, image quality is significantly deteriorated, which leads to serious problems.
Such problems with the residual internal distortion conspicuously occur when a glass ribbon is vibrated in a fluttering manner by ascending airflow generated in an annealing furnace and a posture of the glass ribbon is disturbed. This is because it is difficult to anneal the glass ribbon with a desired temperature distribution when the posture of the glass ribbon is disturbed in the annealing furnace.
Further, such disturbance of the posture of the glass ribbon in the annealing furnace may lead to not only the problems with the residual internal distortion but also a problem of irregular deflection of the glass ribbon, which may significantly deteriorate flatness of glass sheets as finished products.
As a countermeasure, for example, according to the disclosure of Patent Literature 1, convection preventing plates are arranged for preventing ascending airflow in the annealing furnace. However, gaps between the convection preventing plates and the glass ribbon cannot be completely closed, and hence the ascending airflow has much influence on the glass ribbon. In addition, when the glass ribbon and the convection preventing plates are brought close to each other, there may arise an additional problem that the glass ribbon is broken by abutment against the convection preventing plates when the ascending airflow disturbs the posture of the glass ribbon by vibrating the glass ribbon.
Further, according to the disclosure of Patent Literature 2, air pressure of an atmosphere on an outside of the annealing furnace is increased to prevent generation of the ascending airflow in the annealing furnace without use of the convection preventing plates. However, even when such a countermeasure is taken, it is difficult to completely prevent the ascending airflow.
Note that, according to the disclosure of Patent Literature 3, although the above-mentioned ascending airflow is not directly coped with, the glass ribbon caused to flow downward from the forming body is actively curved into a partial cylindrical surface shape over a width direction.